Method of and apparatus for correcting color of print medium, and proofer used therein

0001This is a continuation-in-part of application Ser. No. 09/210,392 filed Dec. 14, 1998 (abandoned), the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

00021. Field of the Invention

0003The present invention relates to a method of and an apparatus for correcting the color of a print medium and a proofer such as a color printer or the like, in a proof generating system which generates a color proof comprising a color image printed on a proof sheet with the proofer, before a colored printed material is produced using a print sheet by a color printing machine such as a rotary press or the like.

00042. Description of the Related Art

0005It has heretofore been customary to generate a color proof for examining colors and making color corrections with a proofer such as a color printer or the like before a colored printed material with a color image in the form of a halftone-dot image printed on a print sheet as a final product is produced by a color printing machine.

0006The proofer is used because it does not require films and printing plates (presensitized plates) to be produced and can easily generate a plurality of hard copies or color proofs within a short period of time.

0007For forming a color image on a proof sheet, image data depending on a device such as a printing machine, a camera, an image sensor, a CRT, an LED, etc., e.g., C, M, Y, K (cyan, magenta, yellow, black) image data, are converted to calorimetric data independent of a device, e.g., X, Y, Z (stimulus value) data, according to a standard printing profile (a four-dimensional lookup table or the like) given by a printer manufacturer or the like. Then, the device-independent calorimetric data are converted to device-dependent image data for a color printer, e.g., R, G, B (red, green, blue) image data, according to a proofer profile, e.g., a printer profile (a three-dimensional lookup table). Using the device-dependent image data, a color proof with a color image formed on a proof sheet is generated by a color printer which is also referred to as a proof printer. In this manner, the colors of a printed material to be produced by the printing machine can be simulated by the color proof thus generated before the printed material is actually produced.

0008However, it often happens for the standard printing profile to fail to match printing properties, i.e., printing conditions depending on inks, papers, and printing machine characteristics, of the printing machine which will actually be used to print the color image.

0009Specifically, the printing conditions vary depending on many parameters which include not only inks, papers, printing machine types, but also production lots and ambient temperatures even when the same inks, paper, and printing machine are used. It is impossible for the standard printing profile to fully match actual printing conditions used by the user, i.e., desired printing conditions. It is therefore necessary to adjust the printing profile according to actual printing conditions, i.e., desired printing conditions, rather than standard printing conditions.

0010Inks and papers that are in general use are limited to certain types. Therefore, solid ink colors and paper colors are relatively stable even under different printing conditions. However, intermediate color tones may vary greatly depending on conditions in which color images are printed by an actual printing machine. For adjusting such intermediate color tones, it has been the practice to insert one-dimensional lookup tables for the respective colors C, M, Y, K to adjust C, M, Y, K tone curves (also referred to as gradation characteristics or dot gains) prior to the four-dimensional lookup table as the standard printing profile, and convert the respective colors C, M, Y, K to C′, M′, Y′, K′, respectively, for color correction.

0011If the proofer is a CMYK printer capable of using print sheets for finally printing color images thereon, then when such print sheets of the same type and lot as will be used by the actual printing machine are used by the proofer, color images printed on print proofs by the proofer can be confirmed as reflecting finally printed color images substantially exactly.

0012However, if sheets used by the proofer are of the type for exclusive use by the proofer and are different from print sheets according to the standard printing profile for use by the actual printing machine, then adjustments need to be made because the color of the print sheets themselves does not match the gradation (dot gain) of highlights (lightly tinted areas). Such adjustments can be made to a certain extent by adjusting the tone curves with the one-dimensional lookup tables. However, the sheet color can only be adjusted in principle by increasing the amount of inks. If the sheet color is adjusted, then not only the sheet color, but also the dot gain of lightly tinted areas or highlights is varied, and it is not clear how the dot gain of lightly tinted areas or highlights is to be adjusted.

0013According to the conventional practice in which proofs are produced by the proofer using dedicated sheets, if the color of print sheets to be used by the actual printing machine is lighter than the color of print sheets according to the standard printing profile, then it is impossible to adjust the dot gain with the sheet color.

SUMMARY OF THE INVENTION

0014It is therefore an object of the present invention to provide a method of and an apparatus for correcting the color of sheets, more generally, the color of a print medium into a lighter tint, and correcting the color of a print medium, i.e., the difference between the color of a standard print medium and the color of a desired print medium, without affecting the dot gain.

0015Another object of the present invention is to provide a proofer for generating a color proof having good color reproduction, on a proof print medium having the color different from the color of the desired print medium.

0016The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiments of the present invention is shown by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

0017<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of a proof generating system to which the principles of the present invention are applied;

0018<figref idref="DRAWINGS">FIG. 2</figref> is a view of a color chart, which is a printed image outputted by a printer, carrying patches that represent colorimetric values L*a*b* of the color of a print sheet to be actually printed and values which vary successively slightly from the colorimetric values L*a*b*;

0019<figref idref="DRAWINGS">FIG. 3</figref> is a view showing a comparison between the colors of the color patches on the color chart shown in <figref idref="DRAWINGS">FIG. 2</figref> and the color of a print sheet to be actually printed; and

0020<figref idref="DRAWINGS">FIG. 4</figref> is a view of a color chart, in which a color of a central color patch in <figref idref="DRAWINGS">FIG. 2</figref> is same as a color of a color sheet and colors of remaining color patches are changed correspondingly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0021As shown in <figref idref="DRAWINGS">FIG. 1</figref>, a proof generating system <b>10</b> to which the principles of the present invention are applied has a color converter <b>12</b> in the form of a computer. The color converter <b>12</b> converts image data depending on an input device, e.g., C, M, Y, K image data Iin=Iin (C, M, Y, K) to be printed, each of halftone-dot % data, to device-dependent image data, e.g., R, G, B image data Iout=Iout (R, G, B), and outputs the R, G, B image data to an image output device as a proofer body, e.g., a printer <b>14</b>. In this embodiment, the proofer comprises the color converter <b>12</b> and the printer <b>14</b> as the proofer body.

0022The C, M, Y, K image data Iin are image data generated by a color separator <b>18</b> which converts R, G, B image data generated by an image input device, e.g., a scanner <b>16</b>, according to a three-color to four-color conversion process (RGB→CMYK) to suit printing properties. The color separator <b>18</b> makes known color corrections, gradation corrections, and other corrections, which will not be described in detail below as they do not have a direct bearing on the present invention.

0023The color converter <b>12</b> connected to an output terminal of the color separator <b>18</b> has one-dimensional conversion lookup tables (LUTs) <b>21</b> through <b>24</b> for effecting gradation conversion on each of the colors C, M, Y, K of the C, M, Y, K image data Iin from the color separator <b>18</b>, and a standard printing profile (given color converting means) <b>26</b> for converting gradation-converted image data Iin′ (C′, M′, Y′, K′) from the LUTs <b>21</b> through <b>24</b> to X, Y, Z colorimetric data Icv=Icv (X, Y, Z) as first calorimetric data which are device-independent image data.

0024The color converter <b>12</b> also has one-dimensional conversion LUTs (color correcting means, also mentioned as a color adjusting function, a color adjusting block, or a color adjuster) <b>31</b> through <b>33</b> for effecting colorimetric conversion on the X, Y, Z colorimetric data Icv to second colorimetric data Icv′=Icv′ (X′, Y′, Z′).

0025The color converter <b>12</b> further has a printer profile (color converting means) <b>36</b> for converting the second colorimetric data Icv′ to the R, G, B image data Iout=Iout (R, G, B).

0026When the R, G, B image data Iout are supplied to the printer <b>14</b>, the printer <b>14</b> generates a proof <b>42</b> which carries a color image <b>39</b> on a color sheet <b>40</b> that is a dedicated proof print medium.

0027The printer <b>14</b> may comprise a laser beam printer which scans the color sheet <b>40</b> with R, G, B laser beams intensity-modulated by the R, G, B image data Iout to record latent images, and develops the recorded latent images into visible R, G, B images. The color sheet <b>40</b> hereinafter refers to a developed color sheet.

0028Processes of generating the LUTs <b>21</b> through <b>24</b> for correcting gradation (dot gain) characteristics, the standard printing profile <b>26</b>, the calorimetric conversion LUTs <b>31</b> through <b>33</b>, and the printer profile <b>36</b> will be described below.

0029The printer profile <b>36</b> is given by a printer manufacturer or the like. For generating the printer profile <b>36</b>, color patches of combinations of the colors R, G, B of the R, G, B image data Iout, each ranging from a zero value to a maximum value, are produced on the color sheet <b>40</b>, and measured by a calorimeter to determine X, Y, Z calorimetric data. Then, a conversion table between the R, G, B image data and the X, Y, Z calorimetric data is generated. The generated conversion table is inversely converted and interpolated, if necessary, thereby producing a LUT as the printer profile <b>36</b> for converting X, Y, Z calorimetric data (the calorimetric data Icv′ in <figref idref="DRAWINGS">FIG. 1</figref>) to R, G, B image data Iout.

0030The printing profile <b>26</b> is also given by a printer manufacturer or the like. The printing profile <b>26</b> is a colorimetric table of values of C, M, Y, K halftone-dot % data at certain intervals. For generating a colorimetric table of values of C, M, Y, K halftone-dot % data at intervals of 10%, for example, it is necessary to determine a total of 11⁴=14641 colorimetric values of the four colors C, M, Y, K at 0, 10, . . . , 100 halftone-dot %. Actually, however, several hundred representative colors of these 14641 colorimetric values are printed as a chart on a standard print sheet by a standard printing machine, and then calorimetrically measured to generate a colorimetric table of values of C, M, Y, K halftone-dot % data at intervals of 10% as the standard printing profile <b>26</b>, which is a nonlinear four-dimensional LUT for converting the image data Iin′ (C′, M′, Y′, K′) to the colorimetric data Icv (X, Y, Z).

0031When halftone-dot % data C′, M′, Y′, K′ of the image data Iin′ (C′, M′, Y′, K′) are supplied to the printing profile <b>26</b>, the printing profile <b>26</b> converts the halftone-dot % data C′, M′, Y′, K′ to the X, Y, Z colorimetric data Icv=Icv (X, Y, Z) as the first colorimetric data.

0032As described above, the printing profile <b>26</b> requires a very large scale of colorimetric operations to be carried out for making itself highly accurate for color conversion. The printing profile <b>26</b> is provided as a standard printing profile by the printer manufacturer. The standard printing profile <b>26</b> is designed to cover different papers including art paper, coat paper, mat paper, and wood free paper, two or three types of ink each having a high market share, and a printing machine having a high market share, which is supposed to operate at normal temperature.

0033The LUTs <b>21</b> through <b>24</b> for correcting gradation (dot gain) characteristics serve to convert C, M, Y, K halftone-dot % data of the C, M, Y, K image data Iin {Iin=Iin (C, M, Y, K)} to C′, M′, Y′, K′ halftone-dot % data of the gradation-converted image data Iin′ {Iin′ =Iin′ (C′, M′, Y′, K′)}.

0034As described above with respect to the related art, since inks and papers that are in general use are limited to certain types, solid ink colors and paper colors are relatively stable even under different printing conditions.

0035However, intermediate color tones may vary greatly depending on conditions in which color images are printed by an actual printing machine. For adjusting such intermediate color tones, it is necessary to insert one-dimensional C, M, Y, K LUTs <b>21</b> through <b>24</b> for adjusting C, M, Y, K tone curves (dot gains) prior to the four-dimensional LUT as the standard printing profile <b>26</b>, and convert the respective colors C, M, Y, K to C′, M′, Y′, K′, respectively, for color correction.

0036For example, if the tone curves (tone curves under desired printing conditions) of a printing machine to be used as the printer <b>14</b> to produce the proof <b>14</b> are more <b>15</b> upwardly convex than the tone curves of the given standard printing profile <b>26</b> (which normally agree with a straight line y=x in an orthogonal plane xy at values of 0 and 100%, but are upwardly convex with respect to the straight line y=x in an intermediate section between the values of 0 and 100%), then the LUTs <b>21</b> through <b>24</b> may incorporate upwardly convex curves to compensate for the difference between the tone curves. Conversely, if the tone curves of a printing machine to be used as the printer <b>14</b> are more downwardly convex than the tone curves of the given standard printing profile <b>26</b>, then the LUTs <b>21</b> through <b>24</b> may incorporate downwardly convex curves to compensate for the difference between the tone curves.

0037Even with the corrective one-dimensional tone curves incorporated in the LUTs <b>21</b> through <b>24</b>, if the color of print sheets to be used actually is lighter than the color of print sheets according to the standard printing profile <b>26</b> (calorimetric values X, Y, Z outputted from the standard printing profile <b>26</b> when the input image data Iin′ have values of C′=M′=Y′=K′=0), then the gradations cannot accurately be converted in highlights (tint areas of the intermediate color tones) including the sheet color.

0038In order to correct the sheet color into a lighter tint, the calorimetric conversion LUTs <b>31</b> through <b>33</b> are employed. For generating the colorimetric conversion LUTs <b>31</b> through <b>33</b>, the input image data Iin′ having values Iin′=0 (C′=M′=Y′=K′) are supplied to the printing profile <b>26</b>, which output X, Y, Z colorimetric values X<b>0</b>, Y<b>0</b>, Z<b>0</b>.

0039The X, Y, Z colorimetric values X<b>0</b>, Y<b>0</b>, Z<b>0</b> are then mathematically converted to calorimetric values L*a*b* (hereinafter referred to as calorimetric values of the sheet color according to the printing profile <b>26</b>) in a CIELAB color space according to a known conversion formula.

0040Thereafter, each of the calorimetric values L*a*b* of the sheet color according to the printing profile <b>26</b> is varied about itself. Specifically, the value of L* relative to luminance is slightly varied about the calorimetric value L*, thereby to calculate values of L*±ΔL, L*±2ΔL. Similarly, the value of a* substantially relative to red is slightly varied about the calorimetric value a*, thereby to calculate values of a*±Δa, a*±2Δa. Furthermore, the value of b* substantially relative to yellow is slightly varied about the colorimetric value b*, thereby to calculate values of b*±Δb, b*±2Δb.

0041Then, the colorimetric value L* of the sheet color according to the printing profile <b>26</b> and the calculated values of L*±ΔL, L*±2ΔL, the calorimetric value a* of the sheet color according to the printing profile <b>26</b> and the calculated values of a*±Δa, a*±2Δa, the colorimetric value b* of the sheet color according to the printing profile <b>26</b> and the calculated values of b*±Δb, b*±2Δb are converted to colorimetric data X, Y, Z according to a known conversion formula.

0042The colorimetric data X, Y, Z thus obtained comprise all combinations of the colorimetric values. Combinations about (L*, a*, b*) are 25 combinations including one combination of (L*, a*, b*), two combinations of (L*, a*±Δa, b*), two combinations of (L*, a* ±2Δa, b*), two combinations of (L*, a*, b*±Δb), two combinations of (L*, a*, b*±Δb), four combinations of (L*, a*±Δa, b*±Δb), four combinations of (L*, a*±Δa, b*±2Δb), four combinations of (L*, a*±2Δa, b*±Δb), and four combinations of (L*, a*±2Δa, b*±2Δb). With an exemplary collection of five color charts, there are a total of 125 combinations.

0043The 125 combinations of Lab data are converted to X, Y, Z data. The converted X, Y, Z data, as the second colorimetric data Icv′=Icv′ (X′, Y′, Z′), are converted to the R, G, B image data Iout {Iout=(R, G, B)} by the printer profile <b>36</b>. The R, G, B image data Iout are then arrayed in a predetermined sequence, and supplied to the printer <b>14</b>, which produces a color chart <b>42</b>A composed of an array of color patch groups <b>61</b> through <b>65</b>. The color chart <b>42</b>A shows that color patches are arranged in respective cross sections across L*-axes having different L* values (for a*−b* planes) in three-dimensional calorimetric values L*a*b*.

0044<figref idref="DRAWINGS">FIG. 2</figref> schematically shows the color chart <b>42</b>A. In <figref idref="DRAWINGS">FIG. 2</figref>, the colors of the color patches vary at small intervals of ΔL, Δa, Δb in the CIELAB color space, rather than the CIEXYZ, because they are believed to match human vision characteristics more effectively than the values X, Y, Z.

0045As shown in <figref idref="DRAWINGS">FIG. 2</figref>, the color chart <b>42</b>A comprises a color sheet <b>40</b> carrying the five color patch groups <b>61</b> through <b>65</b> composed of 5×25=125 color patches. A color patch <b>73</b>, represented by ▪, at the center of the central color path group <b>63</b>, for example, is formed of the colorimetric values L*a*b* of the sheet color according to the printing profile <b>26</b>. Color patches positioned on the right of the color patch <b>73</b> have colorimetric values incremented by +Δa in the rightward direction. Therefore, a color patch <b>74</b>, for example, corresponds to calorimetric values (L*, a*+2Δa, b*+Δb). As shown in <figref idref="DRAWINGS">FIG. 3</figref>, a print sheet <b>80</b> to be actually used for printing (corresponding to desired printing conditions) is prepared, and partly superposed on the color patches of the color patch groups <b>61</b> through <b>65</b>. The user then visually compares the color of the print sheet <b>80</b> and the color patches on the color chart <b>42</b>A with each other to confirm a color difference.

0046Inasmuch as the color of the color sheet <b>40</b> is lighter than the color of the print sheet <b>80</b>, it is possible for the user to easily locate a color (color patch) that is substantially the same as the color of the print sheet <b>80</b> through a visual observation. When the user spots a color patch whose color cannot be distinguished from the color of the print sheet <b>80</b>, the colorimetric data of the spotted color patch, stated otherwise, the input values supplied to the printer profile <b>36</b> (either of the X, Y, Z data converted from the 125 combinations of the Lab data) are regarded as colorimetric data Xα, Yα, Zα.

0047If the colorimetric values of the sheet color according to the printing profile <b>26</b> are X, Y, Z colorimetric values X<b>0</b>, Y<b>0</b>, Z<b>0</b> converted from the colorimetric values L*a*b* of the sheet color according to the printing profile <b>26</b>, then since the X, Y, Z colorimetric values are generally linear, gradients of Xα/X<b>0</b>, Yα/Y<b>0</b>, Zα/Z<b>0</b> are incorporated in the respective LUTs <b>31</b> through <b>33</b>.

0048Therefore, the calorimetric data X, for example, supplied to the LUT <b>31</b> is converted thereby to calorimetric data X′ (=X×Xα/X<b>0</b>), and the calorimetric data Y, Z are also converted thereby to colorimetric data Y′ (=Y×Yα/Y<b>0</b>) and Z′ (=Z×Zα/Z<b>0</b>). Based on the converted calorimetric data X′, Y′, Z′, the gradation (dot gain) in the vicinity of highlights of the image <b>39</b> on the color sheet <b>40</b> of the proof <b>42</b> produced by the printer <b>14</b> can be corrected.

0049If the user has a colorimeter, then it is not necessary to produce the color chart <b>42</b>A, but the user may colorimetrically measure the color of the print sheet <b>80</b> to be used actually, and use the produced colorimetric data as the above colorimetric data Xα, Yα, Zα.

0050As shown in <figref idref="DRAWINGS">FIG. 4</figref>, the color chart <b>42</b>B may comprise a central color patch <b>73</b>B having a same color as the one of the color sheet <b>40</b> (white color of the background) when the color of the print sheet <b>80</b> is determined by partly superposing the print sheet <b>80</b> on the color chart. Accordingly, directional relationships of the color changes from the paper color (the color without correction) can be easily understood.

0051In this case, when colorimetric values of the color paper <b>40</b> are assumed as (L*1, a*1, b*1), a color patch <b>74</b>B, for example, in five color patch groups <b>61</b>B-<b>65</b>B corresponds to colorimetric values (L*1, a*1+2Δa, b*1+Δb).

0052Further, when a color difference between two closest color patches (e.g., between a color patch <b>73</b> and the eight color patches therearound) is assumed as ΔE in the color chart <b>42</b>A, the color difference ΔE preferably has a value from 1.5 to 2, for allowing the user to visually distinguish between two closest color patches (also applicable to color patches in color chart <b>42</b>B).

0053When outputting color chart <b>42</b>A or the like in the above embodiment, increment/decrement intervals of calorimetric values L*a*b* are set to be values of ±Δ, ±2Δ with respect to values L*a*b* as values of Δ multiplied by an integral number. However, the increment/decrement intervals may be adjusted or set to be values of ±1.5Δa as values of 66 multiplied by a real number, for example, which are in the range of values between ±Δ and ±2Δ, respectively, for outputting a color chart. The increment/decrement intervals of values L*a*b* can also be set independently with each other, e.g., ±1.5ΔL for the value L*, ±Δa for the value a*, and ±2Δb for the value b*.

0054According to this embodiment, as described above, the one-dimensional colorimetric conversion LUTs <b>31</b> through <b>33</b> for adjusting the sheet color are inserted behind the standard printing profile <b>26</b>. With this arrangement, first, if a calorimeter is available to the user, then the user can directly correct the sheet color based on colorimetric values of the print sheet <b>80</b> which are obtained by the calorimeter. Second, the sheet color can be corrected into a tint lighter than the color sheet according to the standard printing profile <b>26</b> up to the level of lightness of the color of the color sheet <b>40</b>. Third, tint areas of the intermediate color tones can appropriately be corrected while keeping the dot gain relationship without having to adjust the tone curves.

0055The above second and third advantages are independent of each other such that tint areas of the intermediate color tones can appropriately be corrected while keeping the dot gain relationship no matter which direction the sheet color may be varied in.

0056As described above, the dot gain of the intermediate color tones may be corrected by the one-dimensional C, M, Y, K LUTs <b>21</b> through <b>24</b> placed in front of the printing profile <b>26</b>.

0057In the above embodiment, since the sheet color, the dot gain in tint areas of the intermediate color tones, and the dot gain in other areas of the intermediate color tones can be corrected, the dot gain can be corrected in all areas of the intermediate color tones. Consequently, the sheet color may be corrected to reproduce an image whose colors are accurately representative of those of a printed image, on the color sheet <b>40</b>.

0058Therefore, even if the color of a print medium to be used under desired printing conditions is lighter than the color of a print medium used under standard printing conditions, the color can easily be corrected into a lighter color according to calculations without affecting the dot gain.

0059The principles of the present invention are applicable to a proof generating system for generating a proof with an image output device when a printed material is to be produced using a desired print medium (which may typically be, but not limited to, a sheet of paper) different from a standard print medium (which may typically be, but not limited to, a sheet of paper).

0060According to the present invention, furthermore, the color converting means given to a standard print medium converts device-dependent image data to first colorimetric data, and the color correcting means for correcting the difference between the color of a desired print medium and the standard print medium converts the first calorimetric data to second colorimetric data. Based on the second calorimetric data, the image output device produces a proof on which the difference between the color of the desired print medium and the standard print medium has been corrected.

0061Because the color is calorimetrically corrected, a corrective range that is achieved is wide. If the color of the standard print medium produced by the color converting means is different from the color of the desired print medium to be used, e.g., even if the color of the desired print medium is lighter than the color of the standard print medium, the difference between those colors may be corrected.

0062Before the device-dependent image data are converted to the first colorimetric data by the color converting means, the device-dependent image data are converted in gradation with respect to each color in order to match desired printing conditions for thereby correcting the intermediate color tones, and the sheet color and tints in highlights can be corrected by colorimetric corrections effected by the color converting means.

0063The color correcting means may be generated by outputting color patches, whose calorimetric values are varied in a colorimetric color space about the color of the standard print medium, on a proof medium with the image output device, and visually comparing the color of the desired print medium with the colors of the color patches on the proof medium.

0064The color correcting means may alternatively be generated by calorimetrically measuring the color of the desired print medium.

0065In the color converter <b>12</b> of the above embodiment, at least the printing profile <b>26</b>, the conversion LUTs <b>31</b> through <b>33</b> for effecting calorimetric conversion, and the printer profile <b>36</b> may be combined to be a synthetic LUT (combined LUT) <b>37</b> as one synthetic color converting means (combined color converting means). This configuration realizes fast conversion process in the color converter <b>12</b>.

0066Although a certain preferred embodiment of the present invention has been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.